Fuel cell power plants offer a highly efficient and environmentally clean system for power generation. In the operation of such power plants, hydrogen is used as a fuel and is usually derived from a hydrocarbon feedstock (e.g., natural gas or naphtha) by steam reforming in a separate unit external to the power plant. This is referred to as external reforming and is described in U.S. Pat. No. 3,909,299.
Another technique for generating hydrogen is to reform the hydrocarbon fuel directly within the fuel cell itself and, in particular, within the fuel cell anode compartment. This is referred to as internal reforming and is described in U.S. Pat. Nos. 4,182,795 and 4,877,693. Internal reforming has attracted considerable attention because it offers certain advantages in comparison with external reforming. These advantages include the following: (1) high overall efficiency of the fuel cell system due to on-site consumption of heat evolved in the cell reaction by the endothermic reforming reaction; (2) on-site supply of steam required for the steam-reforming reaction via the anode reaction product; (3) conversion approaching 100% by the consumption of hydrogen by the anode reaction; and (4) byproduct heat removal.
Internal reforming, however, requires special adaptation of the fuel cell power plant to allow the appropriate catalyst to be incorporated inside the anode compartments of the fuel cells used in the fuel cell stack of the plant. FIG. 1 shows a typical fuel cell anode compartment 1 of such a fuel cell. As shown, the anode compartment comprises a separator plate 2 for isolating fuel from the oxidant stream of the neighboring fuel cell, an anode electrode 3 for providing electrochemical reaction sites, and an anode current collector 4, shown as a corrugated plate, for conducting electronic current from the anode electrode and providing a flow path for the gaseous fuel stream. The anode current collector 4 is separated from the anode electrode 3 by an electrolyte barrier 5 and is loaded with a reforming catalyst 6 for converting hydrocarbon feedstock to hydrogen. Also shown in FIG. 1, is the cathode electrode 7 and the cathode current collector 8 of the neighboring cell.
The reforming catalyst 6 is usually available in various compacted, solid shapes such as tablet, pellet, rod, ring or sphere form. Typical techniques for incorporating these types of catalysts in the corrugated space of the anode current collector are described in U.S. Pat. No. 4,788,110. However, there are certain disadvantages to these techniques. Due to the small size of the catalyst particles, handling the particles during assembly is cumbersome and difficult to automate and, therefore, not cost effective. Also, loading uniformity from the fuel cell power plant to fuel cell power plant is hard to achieve. Another difficulty is catalyst shifting and spilling during assembly, handling, transportation and operation.
U.S. Pat. No. 5,660,941, assigned to the same assignee hereof discloses a type of catalyst which avoids some of these difficulties. The catalyst described in this application is in the form of a thin sheet of catalyst material having openings throughout its surface. In particular, the catalyst openings and the pitch of the openings are designed to match the anode current collector corrugations or “legs.”
The catalyst sheet can thus be placed on the current collector, and the sheet held in position by the legs sticking out through the matched openings. As can be appreciated, use of the catalyst sheet avoids catalyst handling problems attendant the use of catalyst particles. However, fabricating a catalyst sheet with the required openings is difficult and may not be as cost effective as is desired.
Another concern facing internal reforming fuel cell power plants, particularly carbonate fuel power plants, is how and where to store sufficient electrolyte to maintain adequate inventory over the desired life of the plant. This may extend to 40,000 to 50,000 hours of use. In fuel cell stacks utilizing carbonate fuel cell power plants, a melted carbonate is used as the electrolyte and is stored in an inert porous matrix plate.
Besides its electrochemical functions, the melted carbonate electrolyte acts to seal and separate reactants through its liquid capillary forces. Thus, it is essential that the pore volume of the matrix be completely filled with the electrolyte during the fuel cell power plant operation. However, during such operation, the electrolyte is gradually lost through evaporation, creepage and corrosion. Thus, to maintain the sealing function and ionic continuity within the fuel cell power plant, sufficient electrolyte needs to be stored in excess of the minimum needed for initial operation.
Arrangements for ensuring sufficient storage of carbonate electrolyte in fuel cell power plants have been disclosed is several U.S. patents (e.g., U.S. Pat. Nos. 4,035,551, 4,064,322, 4,038,463, 4,548,877, 4,596,751 and 5,468,573). One arrangement employs a thick matrix tile between the anode and the cathode electrodes to store the electrolyte. However, such a thick matrix causes high IR losses.
Another disclosed arrangement uses a thin green tape of carbonate electrolyte particles which are held together by a binder. This electrolyte tape is placed between the electrodes and the electrolyte matrix during the stacking of the fuel cell components. The binder is removed during heat-up of the resultant fuel cell stack, followed by the melting of the carbonate particles in situ so that the carbonate flows into the matrix. However, this arrangement causes the fuel cell stack to be dimensionally unstable during the electrolyte melting which is not desirable for stack mechanical stability.
Another arrangement being considered uses the electrodes of the fuel cell power plant as the electrolyte reservoir. In this case, the storage of the electrolyte is achieved by seeping melted electrolyte directly into the porous electrodes of each of the fuel cells prior to the assembly of the cells into a stack. This appears desirable for a stack whose fuel cells have sufficiently thick electrodes so as to accommodate a large enough amount of electrolyte to ensure that after the electrolyte filling of the matrix of each cell, a sufficient amount of the melted electrolyte will remain in the electrodes to sustain prolonged life. To realize this condition, relatively thick electrodes must be used, increasing stack height as well as cost.
A different technique for solving the electrolyte storage problem is disclosed in U.S. Pat. No. 5,468,573, assigned to the U.S. Government. In this technique, an electrolyte paste is placed in the corrugated space of the cathode current collector of each fuel cell prior to the assembly of the cells into a stack. The electrolyte paste contains electrolyte powder mixed with a removable binder such as petroleum jelly, bees wax or glycerin. This technique is capable of providing sufficient amount of electrolyte in the fuel cell. However, the '573 patent fails to describe any methods for packing the electrolyte inside the corrugated current collector.
Methods which may be feasible for accomplishing this are manual or automatic pressing. However, in using a pressing procedure, the amount and the uniformity of the electrolyte are difficult to control. Additionally, it is extremely difficult to pack the electrolyte paste into the corrugated space without overflowing onto undesired contact surfaces.
It is, therefore, an object of the present invention to provide a catalyst or an electrolyte loaded plate which does not suffer from the above-described disadvantages.
It is a further object of the present invention to provide a catalyst or an electrolyte loaded plate formed to permit ease and efficiency of manufacture.
It is a further object of the invention to provide an apparatus and method for forming a catalyst or electrolyte loaded plate.